Colonists on a distant planet are taken over by alien organisms that control their hosts by inflicting pain. Things take a turn for the worse when Spock is infected by one of the organisms and even his Vulcan ability to control pain isn't sufficient to resist them. The Enterprise crew must come up with a means of destroying the organisms without killing their human hosts.

Recap

The Enterprise is on a course tracing a 200-year old pattern of mass insanity that has moved across the galaxy, leaving devastated civilizations in its wake. They are heading for the next planet, Deneva, and as they approach Kirk tries to contact a private transmitter without success. They pick up a small scout ship approaching the Denevan sun that’s too far away to snag with a tractor beam. They try to warn the pilot off but he appears to be insane. The Enterprise approaches the sun and try to stop him without success, with their own shields overheating. The pilot suddenly declares that he’s free just as the sun consumes his ship. As they head for Deneva, McCoy realizes that Kirk has a brother Sam who works as a scientist on the planet, and is accompanied by his family...

This episode resembles a concept developed by one of the three "Deans" of SF -- Robert Heinlein. The aliens in this episode strongly resemble the aliens central to the Heinlein novel, The Puppet Masters.

Spock: I am a Vulcan, Doctor. Pain is a thing of the mind. The mind can be controlled. You're only half Vulcan.Kirk: What about the human half?Spock: It is proving to be… an inconvenience, but it is manageable.

Kirk: Mr. Spock... your logic, as usual, is inescapable.

Kirk: Mr. Spock, regaining eyesight would be an emotional experience for most. You, I presume, felt nothing?Spock: Quite the contrary, Captain. I had a very strong reaction. My first sight was the face of Dr. McCoy bending over me.McCoy: Hmm... it is a pity brief blindness did not increase your appreciation for beauty, Mr. Spock.

McCoy: (whispering) I said, please don't tell Spock I said he was the best first officer in the fleet.Spock: (overhearing) Why, thank you, Dr. McCoy.Kirk: You've been so concerned about his Vulcan eyes, Doctor, you forgot about his Vulcan ears.

Analysis

What Changed in the Remastered Version
Deneva gets a nice upgrade, and we can see a small ship in the teaser scene, where the Enterprise pursues a Denevan intent on flying into the sun. Hand phaser effects improve slightly, and - a very nice addition - we see the satellites deployed by the Enterprise to irradiate Deneva.

Excised Dialogue
While the removal of dialogue to make room for extra commercials is usually well done, there's an extremely awkward cut in the remastered syndicated version. McCoy says he's going to rig up a pair of protective goggles for Spock when he is exposed to the extreme light. Then Spock sits down and they proceed with the test with no goggles. The dialogue where Spock explains that he can't wear the goggles since the Denevans won't wear goggles is removed.